Mitochondrial DNA (mtDNA) molecules from different Drosophila species differ in size from 15.7 to 19.5 kilobase pairs (kb). These differences are accounted for by differences in size of a region (1.0 to 5.4 kb) which is rich in adenine and thymine (A+T). The relative locations of A+T-rich regions and of restriction enzyme cleavage sites of mtDNA of D. melanogaster and other Drosophila species will be mapped. The mapping information will be used to construct heteroduplexes between mtDNA molecules of different species to determine degrees of homology between A+T-rich regions and other regions of the molecules. Using electron microscopy, restriction enzyme and cloning technology, ultra-centrifugation, thermal melting, base composition analyses and DNA sequencing, we will attempt to elucidate the nucleotides sequences of the A+T-rich regions of different sizes. Using the A+T-rich region and restriction enzyme cleavage sites as markers, we will continue our electron microscope studies of replication of Drosophila mtDNAs. Attempts will be made to fuse tissue culture cells from D. melanogaster and other Drosophila species whose mtDNA molecules differ from those of D. melanogaster, in regard to the size of the A+T-rich region and location of restriction enzyme sites, and to identify recombinant DNA molecules in order to learn about the mechanism by which exchanges between molecules are made.